The present invention relates generally to high temperature ceramic insulating materials and applications to ceramic matrix composites.
Various insulating materials to be used as coatings have been developed to strengthen the resistance of underlying substrates to increased temperatures. Thermal Barrier Coatings (TBCs) are commonly used to protect a machine""s critical components from premature breakdown due to increased temperatures to which the components are exposed. Generally, TBCs extend the life of critical components by reducing the rate of metal waste (through spalling) by oxidation.
A fundamental drawback of TBCs is a limitation in thickness that can be used. This thickness limitation of approximately 0.5 mm is due to manufacturing-induced residual stresses, prohibitive costs, required life of the TBC material, temperature limit of the TBC, and mismatches in the coefficients of thermal expansion of the TBC and the substrate. In addition, microstructure of conventional TBCs (those applied by both air plasma spray and physical vapor deposition) is dictated by process conditions, is limited in versatility, and is prone to dimensional and thermal instability at temperatures greater than 1000xc2x0 C.
Materials comparable to TBCs are fibrous ceramic insulating materials. A major drawback of these materials, however, is that they have low densities which lead to very poor erosion resistance. Therefore, fibrous ceramic insulating materials are inapplicable to high velocity gas flow applications.
Monolithic tiles are another material to be used for protecting critical components in high temperature conditions. These tiles have good erosion resistance and insulating properties, however, they are susceptible to thermal shock damage and catastrophic failure. It is, therefore, desirable to provide insulating materials that can withstand high temperatures without the use of thermal barrier coatings, fibrous ceramic insulating materials, or monolithic ceramic tiles.
Commercially available ceramic matrix composites (CMCs) have many potential applications in high temperature environments. CMCs, however, are limited in their exposure to temperatures near 1200xc2x0 C. for long periods of time. In addition, CMCs cannot be effectively cooled under high temperatures ( greater than 1400xc2x0 C.) or high heat flux conditions because of their lower conductivity than metals and their limitations in cooling arrangements due to manufacturing constraints. It is, therefore, desirable to provide a material that can be used to insulate moderate temperature ceramic matrix composites, is also erosion resistant, thermal shock resistant, and has coefficients of thermal expansion relatively similar to that of CMCs.
European Patent Office publication No. 007,511,04, entitled xe2x80x9cAn Abradable Composition,xe2x80x9d filed Jan. 2, 1997, discloses a ceramic abradable material that can be used to seal ceramic turbine components. This material, however, purportedly has a high temperature capability of only 1300xc2x0 C.
A ceramic insulating composition to be used over a higher strength, lower temperature ceramic for application in high temperature environments is provided. The composition comprises a plurality of hollow oxide-based spheres of various dimensions, a phosphate binder, and at least one oxide filler powder, whereby the phosphate binder partially fills gaps between the spheres and the filler powders. The spheres are situated in the phosphate binder and the filler powders such that each sphere is in contact with at least one other sphere.
The spheres may be any combination of Mullite spheres, Alumina spheres, or stabilized Zirconia spheres. Each of the Mullite spheres has a diameter in the range of approximately 0.1 to approximately 1.8 mm, and preferably approximately 0.8 to approximately 1.4 mm. Each of the Alumina spheres has a diameter in the range of approximately 0.1 to approximately 1.5 mm, and preferably approximately 0.3 to approximately 1 mm, and each of the stabilized Zirconia spheres has a diameter in the range of approximately 0.1 to approximately 1.5 mm, and preferably approximately 0.8 to approximately 1.2 mm.
When only Mullite spheres are used, the spheres have a weight percentage of 32%xc2x110% of the composition. When only Alumina spheres are used, the spheres have a weight percentage of 63%xc2x115% of the composition. When only stabilized Zirconia spheres are used, the spheres have a weight percentage of 58%xc2x115% of the composition. In one preferred embodiment of the composition, the spheres are 20% Mullite spheres by volume and 80% Alumina spheres by volume.
The filler powder may be any combination of Alumina, Mullite, Ceria, or Hafnia. In one preferred embodiment of the composition containing mullite spheres and mullite powder, the mullite spheres have a weight percentage of 32%xc2x110%, the mullite filler powder has a weight percentage of 32%xc2x115%, and the phosphate binder has a weight percentage of 31%xc2x115% of the slurry composition. In this preferred embodiment, when only Mullite is used as the filler powder, the combination of the phosphate binder and the Mullite has a viscosity of approximately 9,000 centipoise.
A method of manufacturing the ceramic insulating composition of the present invention is also provided. The method comprises the following steps: (a) mixing raw materials to form a viscous slurry, the raw materials comprising a phosphate binder and oxide filler powders; (b) adding a predetermined amount of hollow oxide-based spheres to the slurry to create a slurry mixture; (c) cast the mixture into presoaked molds; (d) allow the castings, which have a viscosity, to dry; (e) when the viscosity of the castings is sufficiently high for xe2x80x9cgreenxe2x80x9d bodies to be extracted from the molds with minimal dimensional distortion, remove the xe2x80x9cgreenxe2x80x9d bodies; (f) after the xe2x80x9cgreenxe2x80x9d bodies have been removed, recycle the molds by (i) washing out the leached phosphate by running in water followed by oven drying, and (ii) when fully dry, if the dry weight of the mold is within approximately 1% of the original dry weight, use the mold again to perform another casting; (g) transfer the xe2x80x9cgreenxe2x80x9d bodies to a drying oven to remove free water; (h) fire the casts, evaporating residual free water and thermally transform the phosphate to a refractory bond in the process; and (j) finish machine as required.
In a preferred procedure, step (c) of the method of manufacturing the ceramic insulating composition of the present invention further comprises casting the mixture within approximately 24 hours of being made, step (g) further comprises transferring the xe2x80x9cgreenxe2x80x9d bodies to a drying oven at approximately 80xc2x0 C. to remove free water, step (h) further comprises transferring the casts to the firing oven when the xe2x80x9cgreenxe2x80x9d bodies become stable, and step (i) further comprises the following steps: (i1) begin firing by slowly heating the firing oven to a temperature of approximately 120xc2x0 C.; (i2) dwell increasing the firing oven at a temperature of approximately 120xc2x0 C. until most of the free water is removed by evaporation; (i3) slowly increase the temperature of the firing oven to a temperature of approximately 250xc2x0 C.; (i4) dwell increasing the temperature at a temperature of approximately 250xc2x0 C. until all of the free water is removed by evaporation; and (i5) slowly increase the temperature of the firing oven to a temperature of approximately 1200xc2x0 C. to form a refractory phase of the phosphate.
In another preferred procedure, step (e) further comprises a step after extraction from the molds, of shaping the xe2x80x9cgreenxe2x80x9d bodies to conform to the contour of a mating substrate surface. If performed, this step will achieve near net shaping capability.